Engineering
Float Your Boat
Primary Audience: Families
Description: Place different items in water to see if they sink or float and learn
about buoyancy.
Keywords: Physics, Buoyancy
Materials:
• Objects of various weights, sizes and shapes, including Styrofoam
peanuts, paper clips and pennies
• Crown/Bottle Cap
• Eyedropper
• Cloth Towel
• Large dish tub of water or a small plastic drinking cup for each of the
participants
Instructions:
1. Collect a variety of objects. Discuss what characteristics an object needs
to have in order to float, such as weight, size, type of material, etc.
2. Predict which of your objects will float and which will sink. Place each
item, one by one, into the water. Why do some objects float? How is their
appearance different from the objects that sink?
3. Do you think the crown cap will float? Why or why not? Test your
prediction. You may have to put the cap on the water very carefully, but
you should be able to get it to float.
4. Can you add things to the cap and still have it float? What things do you
think it will be able to hold, how much of each thing will it be able to carry
before it sinks?
5. Try a variety of things such as rice, cereal, grains, twist ties, foam, screws,
paper, etc. How much of each thing is the cap able to hold before it sinks?
Why does the cap sink?
What’s Going On?
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Engineering
Buoyancy is the effect that allows objects such as boats and crown caps to float.
An object will move out or displace water when it is placed in water. It turns out
that the weight of water that is displaced is equal to the weight of the object
placed in the water. Floating is caused by the weight of the water pushing up on
the object. But it's not just the weight that's important. The shape is also
important. Boats and crown caps have spaces with air in them. Air is lighter than
water so that helps the water hold up the cap. As the spaces fill up with water or
other stuff, it no longer helps the water float the object and the object sinks.
People often ask, “Why did the Titanic sink?” Perhaps a better question would
be, “Why did the ship float?” The ship was, after all, made mainly from iron and
steel. Its anchors alone weighed 28 metric tons. (That’s almost 62,000 pounds!)
Steel has a density about eight times that of water, so you would expect a ship
made of steel to sink.
However, if you were to look at a plan of the Titanic, you would discover that
most of its volume was occupied by air. Air has a density of about onethousandth that of water. Therefore, the average density of the ship was less
than the density of water. That’s why the ship floated.
Why did Titanic sink? When the Titanic hit the iceberg, water rushed into the
ship’s hull and displaced the air. The average density of the water and the steel
ship was greater than the density of water. What was the result of this change?
The Titanic sank to the bottom of the Atlantic.
COSI | 333 W. Broad St. | Columbus, OH 43215 | 888.819.COSI | www.cosi.org